Are there limits to man's long-term presence in space?

While manned space flights lasting up to six months have resulted in no physiological changes that have proved detrimental to the health of astronauts, the longer missions envisioned aboard space stations in the future require the further consideration of physiological and psychological well-being for both men and women, and for individuals of more advanced age than astronauts of the past. It is speculated that when permanent manning of spacecraft becomes a reality, the microgravity environment and extensive use of electronics may lead to substantial and permanent physiological, and ultimately evolutionary, changes. Headward shifts of body fluids may, for instance, stimulate greater growth and development of the upper half of the body.